The problem occurred after I switched from a MacBook Pro to a Mac Pro (2013). It is hard to describe. The output sent to the headphone amplifier via USB is chopped. The output sounds like tok-tok-tok-tok. In between the toks the sound seems to be normal. The frequency of the toks is greater than one per second. When I route the sound from Reaper to the built-in Mac Pro headphone connector audio is okay.

I tried to record the problem with Reaper, but the recording sounds correct. It seems to be an issue at the output side.